1. Field of the Invention
This invention relates to systems and methods for calibrating an uncalibrated measuring device.
2. Description of Related Art
Sensors, or more generally, transducers, are used in many disciplines to sense a physical phenomenon and to generate an output signal based on the sensed physical phenomenon. Commonly, such sensors or transducers are calibrated based on known simultaneous measurements of the physical phenomenon. In practice, the output signal from the sensor or transducer is calibrated using a measurement of the physical phenomenon made with another instrument. In general, this second instrument has been calibrated itself by some agreed-upon method and against a specified physical object, physical phenomenon, and/or standard.
In some types of measurements, the sensor or transducer of a measurement device cannot be directly coupled to the physical phenomenon to be measured using that sensor or transducer. Such indirect measurements are dependent upon the coupling of the sensor or transducer to the system in which the physical phenomenon being measured occurs. In this case, calibrating the sensor or transducer is dependent both upon the measurement characteristics of the sensor or transducer and upon the coupling of the sensor or transducer to the system being measured. As a result, indirect measurements often require calibrating the sensor or transducer to additional measurements of the system after the sensor or transducer is connected to the system.
However, when attempting to calibrate a sensor or transducer that makes indirect measurements, it is often difficult, if not impossible, to make, at the same location on the system, both the calibration measurements using that sensor or transducer required to calibrate that sensor or transducer and to make the second set of measurements necessary for calibrating such indirect measurements. This typically, although not always, occurs due to the size of the sensors relative to the system being measured and the practical limitations when making measurements on the system being measured.
For example, due to the size of the sensor or transducer, the size of the secondary measurement device and/or the practical limitations of making blood pressure measurements on living beings, it is generally impossible to measure the blood pressure of a blood vessel within a living being using the sensor or transducer to be calibrated, while making the secondary measurements (discussed above) at the same point on the blood vessel using a second measurement device. For example, the known blood pressure measurements, which are used as the xe2x80x9cadditional measurementsxe2x80x9d for calibrating indirect measurements, are typically obtained from a human being by placing a cuff over the brachial artery of the upper arm. In contrast, a tonometric sensor to be calibrated is placed against the radial artery at the wrist of the human being. Moreover, the blood pressure cuff will very often be placed on an opposite limb from that on which the blood pressure of the human being is being measured using the tonometric sensor. However, it should be appreciated that this inability to make measurements at the same location relative to the system due to physical constraints is not restricted to measuring blood pressure in a living being.
It should also be appreciated that, if the calibration of the sensor or transducer is to be accurate, it is usually necessary that the sensor or transducer being calibrated be exposed to the identical level of the physical phenomenon as that to which the standard measurement device is exposed. If these physical phenomenon cannot be measured both by the sensor or transducer to be calibrated and by the standard measurement device at the same point and if the values of the physical phenomenon are not the same at the two measurement locations, an error in the calibration can result. The values of the physical phenomenon at the two measurement points can differ due to a time delay of the propagation of the physical phenomenon between the two measurement locations and/or due to a distortion of the physical phenomenon between the two measurement locations.
For example, the physical phenomenon of blood pressure in a vascular system of a living being experiences both of these error-inducing characteristics. That is, the propagation of the blood pressure pulse wave through the vascular system has a finite velocity. As a result, the blood pressure measured at a second, downstream location of the vascular system is delayed in time from the blood pressure that occurs at a first, upstream, measurement location of the vascular system. At the same time, as the blood pressure pulse wave propagates through the vascular system, the physiological characteristics of the vascular system of the living being produces distortions in the blood pressure pulse wave that makes the blood pressure pulse wave take different shapes at the first and second measurement locations.
This invention provides systems and methods for calibrating a sensor or transducer that is indirectly coupled to a phenomenon occurring within a system.
This invention separately provides systems and methods for calibrating sensors or transducers that are indirectly coupled to a system where the secondary measurement occurs at a location separated from the location of the measurements obtained by the sensor or transducer to be calibrated.
This invention separately provides systems and methods for calibrating a sensor or transducer relative to a physical phenomenon that is distorted relative to a measurement of that physical phenomenon by a device of known calibration.
This invention separately provides systems and methods for calibrating a sensor or transducer that is indirectly coupled to a physical phenomenon occurring within a living being.
This invention separately provides systems and methods for calibrating a blood pressure transducer that generates an electric signal from a blood pressure signal occurring within a living being.
This invention separately provides systems and methods for calibrating a blood pressure sensor or transducer that senses a blood pressure signal in a living being relative to a separate measurement of the blood pressure signal within the living being taken at a point separated from the location of the blood pressure sensor or transducer to be calibrated.
This invention separately provides systems and methods for determining the calibration parameters of an uncalibrated device in situ using frequency analysis of the naturally occurring variations of the system being sensed using the uncalibrated device.
In various exemplary embodiments of the systems and methods according to this invention, a first transfer function that defines the transformation of a physical phenomenon between a first location and a second location is defined. Next, a value of that transfer function at a particular frequency is determined. Independently, a second transfer function, defining the conversion of the input physical phenomenon to the output signal generated by the sensor or transducer to be calibrated in response to measuring that physical phenomenon, is defined. Additionally, a relationship between the first transfer function and the second transfer function is also defined. The relationship between these two transfer functions is the reciprocal of a calibration coefficient needed to calibrate the uncalibrated sensor or transducer. By obtaining a value for each of the two transfer functions at a particular time, a particular frequency or the like, the calibration coefficient can be obtained.
Independently, the output of the uncalibrated sensor or transducer is based on the calibration coefficient, the input physical phenomenon and a calibration constant. Because the calibration coefficient is known, and because the input and output signal values can be determined or derived, the calibration constant can be determined. By determining the calibration coefficient and calibration constant for the uncalibrated sensor or transducer in situ using frequency analysis of the naturally occurring variations of the system, inaccuracies occurring as a result of using existing time-domain calibration methods can be reduced, or ideally, eliminated.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.